Fix Cube.rolling_window with lazy auxiliary coordinates (#6480)

[gaoflow]

🚀 Pull Request

Description

Closes #6480 (and the duplicate #6639).

:meth:iris.cube.Cube.rolling_window raised a TypeError when the cube had a lazy auxiliary coordinate running along the windowed dimension:

import numpy as np, dask.array as da
from iris.coords import DimCoord, AuxCoord
from iris.cube import Cube
from iris.analysis import MEAN

dimco = DimCoord(np.arange(12), standard_name="time", units="s")
auxco = AuxCoord(da.arange(12), long_name="x")   # lazy
cube = Cube(np.arange(12),
            dim_coords_and_dims=[(dimco, 0)],
            aux_coords_and_dims=[(auxco, 0)])
cube.rolling_window(coord="time", window=3, aggregator=MEAN)

TypeError: '>=' not supported between instances of 'tuple' and 'int'

Cause

When building the new coordinate bounds, rolling_window selects the first and last point of each window with:

new_bounds = new_bounds[:, (0, -1)]

A tuple index is interpreted by Dask as a multidimensional index (and so it tries to use 0 and -1 as indices into successive axes), which fails for a lazy array. NumPy happens to tolerate the tuple here, so the bug only surfaces for lazy coordinates.

Fix

Index with the list [0, -1] instead. NumPy and Dask both treat a list as selecting the first and last column, so the result is identical for real arrays and now also works (and stays lazy) for lazy ones. This is the fix hinted at by @pp-mo on the issue. Both the numeric and string-coordinate branches are updated.

Verification

• New test Test_rolling_window::test_lazy_aux_coord — asserts a lazy aux-coord no longer fails, stays lazy (points and bounds), and equals the real-array result. It fails on main and passes here.
• Manually confirmed identical results for lazy vs. real, numeric vs. string coordinates.
• Full Test_rolling_window and tests/unit/util/test_rolling_window.py suites pass (25 tests).
• ruff check / ruff format clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.15%. Comparing base (746f0a6) to head (bc6142c).

Additional details and impacted files

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##             main    #7123   +/-   ##
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  Coverage   90.15%   90.15%           
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  Files          91       91           
  Lines       24985    24985           
  Branches     4685     4685           
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  Hits        22526    22526           
  Misses       1682     1682           
  Partials      777      777           

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[gaoflow]

Rebased onto current main and resolved the conflict from the towncrier changelog migration:

• kept docs/src/whatsnew/latest.rst as the generated towncrier stub
• moved this PR's whatsnew entry to changelog/7123.bugfix.rst

Local verification:

• git diff --check
• uvx ruff check lib/iris/cube.py lib/iris/tests/unit/cube/test_Cube.py
• uvx ruff format --check lib/iris/cube.py lib/iris/tests/unit/cube/test_Cube.py

I could not run the targeted pytest locally because this environment is missing the Iris runtime dependency cf_units.

[ESadek-MO]

Hi @gaoflow, thanks for this one!

Only a few small changes, mostly preference and consistence rather than any large issues!

[ESadek-MO]

Suggested change

	# (see #6480).
	# (see #6480).
	window = 2

Not drastically important, but keeping related tests aligned helps with readability.

[ESadek-MO]

Suggested change

	res_cube = self.cube.rolling_window("val", iris.analysis.MEAN, 3)
	res_cube = self.cube.rolling_window("val", iris.analysis.MEAN, window, mdtol=0)

[ESadek-MO]

I don't think this comment is needed at all, but if you'd like to keep it I'd shorten it.

[ESadek-MO]

Don't think this is needed, it's implicit in the test.

[gaoflow]

Updated in bc6142cd and rebased onto current main.

Review follow-up:

• shortened the Dask indexing comment in cube.py
• aligned the regression test with the nearby window = 2 / mdtol=0 style
• removed the extra explanatory test comment
• kept the existing changelog/7123.bugfix.rst fragment

Verification after the update:

uv run --with pytest --with pytest-mock --with requests --with filelock --with distributed --with-editable . python -m pytest lib/iris/tests/unit/cube/test_Cube.py::Test_rolling_window::test_lazy_aux_coord -q
ruff check lib/iris/cube.py
ruff format --check lib/iris/cube.py lib/iris/tests/unit/cube/test_Cube.py
uv run --with towncrier towncrier build --draft --version 0.0
git diff --check origin/main...HEAD